1. Field of the Invention
The present invention relates to a method of and apparatus for detecting, among various defects of a transparent sheet, a defect that is caused by distortion such as "knots", "cords" or "striae", unmelted particles or "stones", "bubbles", etc.
2. Description of the Prior Art
A known method utilizes a one-dimensional camera for detecting a shadow caused by interception of light by an internal or surface defect of a transparent sheet as disclosed in Japanese Patent Provisional Publication No. 1-189549. Another known method utilizes a point light source for casting a light through a transparent sheet on a screen and a sensor for detecting a defect in a projected figure as disclosed in Japanese Utility Model Provisional Publication No. 3-27343. A further known method is adapted to observe a row of parallel lines through a transparent sheet so that due to a difference in refraction a defect is represented by distortion of the lines as disclosed in Japanese Patent Provisional Publication Nos. 61-176838 and 61-176839. A further known method utilizes transmission and reflection of a laser beam for detection of a defect of a transparent sheet.
A problem of the prior art methods is that a fleck of dirt, etc. on a transparent sheet is erroneously judged as a defect caused by "knots", "cords", "stones", "bubbles", etc.